In lithographic printing, an inked printing plate contacts and transfers an inked image to a rubber blanket, and then the blanket contacts and transfers the image to the substrate being printed. Lithographic plates have image areas with a relatively oleophilic surface and non-image areas with a relatively hydrophilic surface. In a typical two-fluid lithographic printing process, the plate cylinder first comes in contact with dampening rollers that transfer an aqueous fountain solution that wets out the hydrophilic non-image areas of the plate. The dampened plate then contacts an inking roller, accepting ink only in the oleophilic image areas. The fountain solution helps to keep the ink out of the non-image areas. The press operator must continually monitor the printing process to insure that the correct balance of the fountain solution and the ink is maintained so that the ink adheres to the image areas, but only the image areas, of the plate in order to produce a sharp, well-defined print.
A conventional aqueous fountain solution contains a number of components to enhance performance of the water on the plate. First, the fountain solution includes a plate desensitizer to increase ink repellency from the non-image area. Examples of conventional desensitizers are gum arabic, hydroxypropyl cellulose, and carboxymethyl cellulose. The fountain solution also typically includes a pH buffer. Other common fountain solution components include wetting agents such as isopropanol, glycol ethers, and other water-miscible organic solvents, corrosion inhibitors, fungicides, and antifoaming agents.
The fountain solution is prepared by adding a concentrate containing the desensitizer, wetting agents, and other components to water, generally R.O. or D.I. water. Preparing a fountain solution adds to the expense and preparation time for a printing process. Further, care must be taken to combine the concentrate and water in the right proportions to make an effective fountain solution. In addition, the organic solvents, e.g. wetting agents, in the fountain solution produce unwanted organic emissions.
Van Hunsel et al., U.S. Pat. No. 6,436,176 describes a dampening solution that includes a treated silica sol, water-soluble organic solvents such as alcohols, ethers, polyglycols, and esters, a pH between 3 and 6 obtained with a phosphate salt as buffer, citrate ions, surfactants, thickening agents, preservatives, and chelate compounds.
Matsumoto et al., U.S. Pat. No. 6,312,873 teaches that a fountain solution containing only an aqueous solution of an alkali metal salt of bichromic acid or phosphoric acid or salt or gum arabic were difficult to uniformly wet the non-image area. To overcome this problem, 20-25% isopropanol is added. Isopropanol, however, evaporates too quickly from the fountain and has an odor, and the Matsumoto '873 patent proposes a block copolymer of polypropylene oxide and polyethylene oxide as a replacement.
The industry thus continues to struggle to overcome the problems associated with making and maintaining a complicated fountain solution composition to avoid toning.